The present invention relates to a a shaddow-mask-type color picture tube and, more particularly, to a shadow mask thereof.
In a shadow-mask-type color picture tube, shown in FIG. 1, an envelope formed of glass substantially consists of a rectangular panel 1, a funnel 2 and a neck 3. On an inner surface of the panel 1, for example, a stripe phosphor screen 4 which emits red, green and blue light is provided. On the other hand, in-line electron guns 6, which are linearly arranged along a horizontal axis of the panel 1 and emit three electron beams 10 corresponding to red, green and blue, are provided in the neck 3. A shadow mask 5 having a main surface portion in which a plurality of apertures are formed is disposed adjacent and opposed to the screen 4. A peripheral portion of the shadow mask 5 has a skirt portion 8, which is bent in correspondence with an outer shape of the panel 1. The skirt portion 8 is supported and fixed by a mask frame 7 consisting of a frame having an L-shaped cross-section. Furthermore, the mask frame 7 is engaged through a spring 9 with a pin (not shown), which is buried in an inner wall of the panel 1. In such a color picture tube, the three electron beams 10 emitted from the electron guns 6 are deflected by a deflection apparatus (not shown) provided near the funnel 2 of the outer portion of the envelope. The beams 10 are color-selected by the apertures of the shadow mask 5 while scanning a rectangular region substantially corresponding to the rectangular-shaped panel 1, and respectively and properly bombard the corresponding color-emitting phosphor stripes, thereby forming a color image.
In this case, an effective amount of the electron beam 10 passing through the apertures of the shadow mask 5 is less than 1/3 of the total electron beam emitted from the electron guns 6. The remaining electron beam bombards the shadow mask 5 and is converted into heat energy. For this reason, the shadow mask 5 can be heated to about 80.degree. C. The shadow mask 5 comprises a thin plate having a thickness of 0.1 to 0.3 mm and is formed of cold-rolled steel mainly consisting of iron having a relatively large thermal expansion coefficient of 1.2.times.10.sup.-5 /.degree.C. The mask frame 7, which supports the skirt portion 8 of the shadow mask 5, is formed of the same cold-rolled steel as that of the shadow mask 5 and has a thickness of about 1 mm and an L-shaped cross-section. A surface of the mask frame 7 is oxidized, thereby forming a black oxide layer thereon. Thermal expansion of the shadow mask 5, which is heated by bombardment of the electron beam 10, can easily occur. However, since the peripheral portion of the shadow mask 5 is in contact with the mask frame 7, which has been subjected to darkening and has a large thermal capacity, heat is transferred to the mask frame 7 from the peripheral portion of the shadow mask 5 by radiation and conduction. Therefore, the temperature of the peripheral portion of the shadow mask 5 becomes lower than that of the central portion thereof. For this reason, a so-called doming occurs in which the central portion of the shadow mask 5 is thermally expanded by a greater extent than the peripheral portion thereof. By this doming, the relationship between the position of the apertures of the shadow mask 5 and that of the phosphor stripes formed corresponding to the apertures is changed. Therefore, a landing error occurs in which the electron beams 10 passing through the apertures do not bombard the proper phosphor stripes, resulting in degradation of color purity. Particularly, this doming is considerable at the initial operating phase of the color picture tube.
In addition, when there is a high brightness portion in the image and the portion is stationary for a certain period of time, the shadow mask is locally bombarded by electron beams of high electron current density, causing a local doming of the shadow mask.
With respect to such a doming in the initial operating phase of such a color picture tube, many suggestions have been made relating to prevention of thermal conduction to the shadow mask. For example, in the color picture tube disclosed in U.S. Pat. No. 3,887,828, a porous layer of manganese dioxide is deposited at a side of an electron gun of a shadow mask, and an aluminum layer and a nickel oxide or nickel-iron layer are sequentially formed thereon by vacuum evaporation. In this case, since the thermal conduction coefficient of the porous layer is extremely small, heat caused by bombardment of an electron beam is not transmitted to the shadow mask, but is radiated in a direction away from the shadow mask. For this reason, the temperature increase of the shadow mask can be effectively controlled.
However, this type of shadow mask is not so effective with respect to a local doming although it is effective with respect to a doming in the initial operating phase. In addition, to provide three layers on the shadow mask by vacuum evaporation, a great deal of equipment and long operation time are necessary. The process is therefore extremely undesired from the standpoint of industrial mass production.
Meanwhile, it has been proposed to alleviate miss-landing of electron beams on the side of the screen. For example, U.S. Pat. No. 4,065,695 discloses a structure in which an electron absorbing layer having a low electric conductivity is formed on nonluminous areas of a screen surface free from phosphor. Where the screen having such a structure is employed, a region of the screen where electron beams miss-land on the electron absorbing layer formed on the nonluminous area is also bombarded by electrons so that it is negatively charged. Consequently, local decelerating electric fields are generated between the screen and shadow mask. These electrical fields can serve to correct the trajectory of electron beams that are subject to miss-landing, thus reducing miss-landing. The screen of this structure, however, has the following drawbacks.
In the first place, the formation of an electron absorbing layer on non-luminous areas of the screen surface requires very elaborate equipment and many man-hours. More specifically, to form the electron absorbing layer, a thin precoat layer of polyvinyl alcohol is first formed on the screen surface using a 0.2-% aqueous soluton of polyvinyl alcohol. Then, a layer of a photosensitive suspension of aluminum oxide is formed on the precoat. The photosensitive suspension is prepared by pulverizing 300 g of granular aluminum oxide powder together with 33 g of polyvinyl alcohol, 0.8 g of ammonium bichromate and 1,025 ml of water using a ball mill. The photosensitive suspension becomes water-insoluble when it is exposed to light. The shadow mask with the photosensitive suspension layer is exposed three times using an annular light source having a center at the deflection point of the three electron beams. The exposure is effected for the nonluminous areas of the screen only. Subsequently, the unexposed portions of the photosensitive suspension layer in the water-soluble state are removed by spraying water, whereby an electron absorbing layer is formed on the nonluminous areas of the screen surface free from phosphor. This formation process of the electron absorbing layer requires substantially the same equipment and steps as those when forming luminous areas of screen with phosphors. Besides, if the electron absorbing layer is erroneously formed on the luminous areas as well due to difficient precision, for instance, it will reduce the light output from the luminous areas and, in an extreme case, cause extreme deterioration, as contamination spots on the phosphor screen, of the color picture tube. The structure, therefore, is very poor for industrial mass production from the standpoint of the preparation process and precision.
A second drawback is that a negatively charged portion of the electron absorbing layer, formed between adjacent phosphor regions which emit green, blue, and red light, is found only where electron beams miss-land, that is, it is limited to a very small area, too small to obtain a sufficient decelerating electric field for the electron beam trajectory correction. Further, the decelerating electric field effectively acts on electron beams only in a zone between the shadow mask and electron absorbing layer and very close to the latter, so that it can correct the electron beam trajectory only to a very small extent.